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Adds payment docs and code examples to docs.
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<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
* **Documentation**
* Added comprehensive Payments app guide covering Stripe integration,
subscriptions, and one-time purchases
* Included extensive, multi-language code examples for payment workflows
(JavaScript/TypeScript and Python)
* Added examples for checkout flows, item management, consuming credits,
listing products, and granting products
* Integrated the Payments examples into the central examples collection
and updated docs navigation to include the Payments guide
<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
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